In recent years, there have been proposed image sensors including pixels having different exposure lengths mixed (see e.g. Patent Document 1). In an image sensor including pixels having different exposure lengths mixed, at the time of photographing moving images, all the pixels are read and output in each vertical synchronization period. Accordingly, even in a pixel having a large exposure length, the exposure length cannot be made larger than a vertical synchronization period, so that an SNR of a dark place in a moving image is low.
Methods of improving an SNR include executing two-dimensional noise reduction (2DNR) processing in which a photographed image is subjected to filtering processing in a screen. However, it is difficult to maintain a texture of a subject while reducing noise under strong noise.
Additionally, methods of improving an SNR also include executing three-dimensional noise reduction (3DNR) processing in which noise is reduced by using a photographed image of a past frame. This method is to reduce random noise by weight-averaging an image of a past frame and an image of a present frame in a stationary subject region. Accordingly, when this method is used for a moving subject region, such image deterioration as tailing occurs in a photographed image. Accordingly, complicated processing is required such as conducting determination of a moving subject with respect to a photographed image and preventing weight-averaging from being conducted in a moving subject region.
However, in a dark place under strong noise, it is difficult to precisely determine a moving subject and noise, so that a stationary subject region might be determined to be a moving subject region and noise might not be satisfactorily reduced.